In silicon components, aluminium is often used as contacts to, for example, the base and the emitter. This has the following disadvantages:
Aluminium spiking may occur through thin pn junctions, resulting in short circuits. If pure aluminium is used, silicon from the surface can diffuse into the aluminium, leaving voids or non-homogenous regions below the aluminium contact. If an alloy of aluminium with silicon is used the silicon can be deposited in the contact areas in form of precipitates causing high contact resistance.
To avoid the above mentioned disadvantages which occur when aluminium comes in direct contact with silicon, a barrier layer, for example of TiW, is normally used between the silicon layer and the aluminium. In this case both n type and p type silicon must be doped under the contact surfaces to achieve low contact resistance. To avoid avalanche breakdown between n+ and p+ areas at low voltages, they need to be separated. The distance by which they are separated is important for the characteristics of the component. The separation can be accomplished in several ways. One common way is to implant the dopants through two individual masking layers. The second mask must be aligned to the first mask. Uncertainties in this alignment will always cause some lateral error between the layers.
If the component is to be doped both with acceptors and donors, as is the case with practically all semiconductors, the process of masking must be carried out twice: once masking out the parts which should not be exposed to acceptors before doping with the p dopant, and once masking out the parts which should not be exposed to donors before doping with the n dopant. Also, the masks must be dimensioned and applied very carefully to achieve doping of exactly the desired areas. There is also a risk that the dopant will not be completely restricted to the unprotected areas.
U.S. Pat. No. 4,451,844 discloses a semiconductor device manufactured using a layer of polycrystalline silicone (polysilicon) to cover the active area of the component. The masks used for covering parts of the component while doping are applied on top of the polysilicon, and the component is doped through the layer of polysilicon.
U.S. Pat. No. 5,451,532 describes a process for making a bipolar transistor in which a polysilicon layer is used both as an emitter, which raises above the surface of the transistor, and, together with sidewall spacings, as a mask when doping the base contact, which is placed around the emitter.